It has not been effectively possible to conduct an underwater survey for radionuclides over extensive bottom areas of oceans, lakes or rivers up to present. Radionuclides are generally mostly associated with sediments on the bottom and, as a result, cannot be detected through more than a few inches or feet of water. Gamma surveys of sediments on the bottom are at present done by coring because of the shielding effects of water. Coring is a point sampling approach and does not provide satisfactory coverage without enormous effort.
Sleds with radiation detectors may be used for special investigations since coring is unsuitable to cover large areas. However, sleds are prone to snagging, may tip over or even run upside down when being towed across the bottom from moving boats. Snagging of the sleds could result in a loss of the sled whereas any tipping of the sled results in the detector being shielded from the bottom by the additional depth of water or by the sled itself. This results in highly unreliable readings being obtained.
The majority of sleds presently used for underwater surveys are designed to operate at a predetermined distance above the bottom which makes them unsuitable for gamma surveys of the bottom.
U.S. Pat. No. 4,586,452 shows one type of tow vehicle for underwater sonar and/or TV survey systems wherein the tow vehicle is maintained at a fairly constant distance above the sea floor. A chain is attached to the tow cable at a predetermined distance from the tow vehicle, the chain being of sufficient weight to pull the buoyant tow vehicle downwardly in the water until the chain contacts the sea floor. The chain is then pulled over the sea floor and maintains the tow vehicle at a predetermined distance above the bottom as the vehicle and chain are towed by a vessel.
U.S. Pat. No. 4,426,950 shows another type of tow vehicle for surveying the water bottom by sonars and echo-sounders. That vehicle is maintained at, in this case, a predetermined depth from the surface of the water.
U.S. Pat. No. 4,298,840 shows an apparatus for determining electrical resistivities of various parts of a marine bottom. A flexible logging cable is towed by a vessel along the bottom of a body of water. A number of electrodes are located on the outer walls of the cable at spaced locations along the length of the cable with a first group of electrodes being current supply electrodes and electrodes of another group being measuring electrodes for measuring the potential differences between at least two pairs of electrodes.